The lower the resolution, the more of a load the CPU has. Dropping the render resolution actually decreased performance in this case. That GPU can handle the entire game maxed out at 1440p. Hell, my fiance's 1070ti can handle it maxed out at 1080p. In fact, I'd be willing to bet it can handle RTX on as well at that resolution.

So if you can, try setting a render target at 1440p, turn ALL the options all the way on, putt his refresh back to 144hz, and give the GPU more load. If you haven't already grab MSI Afterburner and look at GPU usage while doing all this.